Not Applicable
Not Applicable
1. Technical Field
This invention relates in general to code division multiple access (CDMA) devices and, more particularly, to a method and apparatus for calculating a dot product for use therein.
2. Description of the Related Art
Spread spectrum communication devices have been used military and commercial devices for many years. These devices use noise-like waves to spread. (cover) the information bits so that transmission bandwidth is much wider than that required for simple point-to-point communication. DS-CDMA (Direct Spreading Code Division Multiple Access) is a form of spread spectrum which uses a conventional communication waveform and a pseudo-noise (PN) sequence; to transmit information. The PN sequence is commonly generated using a Linear Feedback Shift Register (LFSR). Thus, communication channels are separated by means of a pseudo-random modulation that is applied and removed in the digital domain, not on the basis of frequency, like frequency hopping CDMA.
CDMA has some significant advantages over other communication techniques, in particular it has improved capacity and quality as compared to narrowband multiple access wireless technologies.
Direct Spreading (DS) CDMA systems require correlation operations over two long data sequences in real time. One of the sequences is the 2xc3x97M bit wide complex number data and the other is the 2xc3x971 bit complex PN sequence, where M is the data bit width. In correlation calculations, PN bits equal to xe2x80x9c0xe2x80x9d are mapped to xe2x80x9c1xe2x80x9dand PN bits equal to xe2x80x9c1xe2x80x9d are mapped to xe2x80x9cxe2x88x921xe2x80x9d. At a specific offset, the correlation becomes a complex number dot product operation, which can be formulated as:   S  =            ∑              k        =        1            N        ⁢          xe2x80x83        ⁢                  D        ⁡                  (          k          )                    ⁢      PN      *              (        k        )            
where the data D(k)=D(k)I+jD(k)Q) and pseudo-noise sequence PN(k)=PN(k)I+jPN(k)Q, xe2x80x9c*xe2x80x9d is the complex conjugate operator and N is the vector length of the complex data and PN vectors.
In a high quality DS-CDMA system, a parallel dot product operator is needed to make the correlation operation faster and more efficient. Because N is usually a very large number, the hardware implementation of the parallel dot product operator has to be piecewise parallel. This can lead to a large amount of circuit area being devoted to calculation of the dot product.
Therefore a need has arisen for a more efficient method and apparatus for calculating a dot product with a large integration length.
The present invention provides a correlator for performing a dot product operation on bits of a pseudo-noise sequence and respective data words of a data stream. Inversion circuits each receive one of the data words along with an associated pseudo-noise sequence bit and selectively invert bits of the data word responsive to its respective pseudo-noise sequence bit. An adder tree comprising a plurality of adders performs a summation of the outputs of the inversion circuits. The carry-in bit inputs of the adders are coupled to the bits of said pseudo-noise sequence bits.
The present invention provides significant advantages over the prior art. While a dot product over two vectors generally requires L multiplications and L-1 additions, the present invention does not need expensive multiplier of two""s complement numbers as a normal correlator does. The carry-in ports of the adders complete the two""s complement operation, thereby saving an entire level of L adders. Accordingly, gate counts and power consumption are significantly reduced.